British Airways passengers are facing delays after an IT glitch affected check-in desks.
The airline apologised to customers, saying its IT teams were "working to resolve this issue".
British Airways told customers that some flights were cancelled on Monday "due to operational reasons".
It said in a statement: "We are checking in customers at Heathrow and Gatwick Airports this morning, although it is taking longer than usual."
"We are sorry for the delay to their journeys," the airline added.
Customers were encouraged to check in online before they reach the airport.
On Monday customers in the US and Canada reported delays at several airports due to IT problems.
People flying from San Francisco, Washington DC and Atlanta reported long delays on social media.
Twitter user John Bevir wrote: "Huge computer issue affecting British Airways across USA. Friend at #Dulles tells me pilots by gate but passengers still trying to check in!".
Ewan Crawford, of Glasgow, said he was at Chicago O'Hare International Airport. He tweeted: "Never a good sign when they deliver water to the gate! Waiting at ORD for @British-Airways 296. Worldwide computer outage apparently! Hmm."
Matthew Walker, a financial analyst from London, waited for more than two hours to board his flight to Heathrow.
Though he had already checked in online, he said that BA staff could not access their computers to see which passengers had gone through security.
He said: "People were lining up, some had already checked in and got through security, but others, when this thing happened, whatever it is, were stuck in the check-in queue.
"So they (the staff) have the problem that they didn't know who had already gone through the gate because all the systems literally just had a meltdown."